No model for weather or climate is reliable out to a few decades. Uncertainty is an exponential function of time in chaotic systems. One good super volcano and the earth would plunge into a glacial period. A series of large volcanoes over many years would keep us there for a long, long time. We have absolutely no way to predict supervolcanoes or general volcanic activity over the next several decades.
For an example of what can happen from simply a large volcanic eruption, see the Year Without a Summer in 1816: https://en.m.wikipedia.org/wiki/Year_Without_a_Summer
But that was not a super volcano. It was just one large eruption.
We have only increased by about 1 degree in the past 100 years. We are still cooler than the Medieval Warm Period when Vikings were farming parts of Greenland, and cooler than the Roman Warm Period, when they were growing wine grapes in Scotland. It's doubtful that CO2 increase of even doubling would cause catastrophic crop failure. After all, the CO2 levels were thousands of PPM for most of Earth's history, and plants evolved under those conditions before the Pliestocene. Dinosaurs were very large creatures that were supported by a rich ecosystem of plant life. CO2 is plant food. Only at extremely high levels of 7000+ ppm (20x modern levels) has there ever been any geological evidence of catastrophe and even that event is hard to disentangle cause and effect.